ITMI952137A1 - MULTI-LAYER SHRINK FILM AND PROCEDURE FOR ITS PRODUCTION - Google Patents

Abstract

Multilayer shrink film, with good optical and mechanical properties and improved shrinkability properties, comprising an inner layer comprising a greater quantity of an ethylene-propylene-butene-1 copolymer, or a mixture of these, and two outer layers, having a different composition from that of said inner layer, each of which comprising a greater quantity of an ethylene-propylene copolymer, of an ethylene-propylene-butene-1 terpolymer, or of a mixture of these, optionally in a mixture with a smaller quantity of homopolymer polypropylene.

Description

Description of the industrial invention entitled:

"New multi-layer shrink film and process for its production"

The present invention relates to a new multilayer, heat-shrink film, having good optical and mechanical properties and improved heat-shrinking characteristics, particularly suitable for packaging food and non-food products.

Thermoplastic shrink films are useful for many applications in the food and non-food packaging industry.

In general terms, the packaging of food and non-food products by means of a heat-shrinkable thermoplastic film includes positioning the shrink-wrapped packaging material partially or completely around a product, removing, if necessary, excess air, sealing the thermoplastic material to itself or at the edges of the support that contains the product to be packaged or otherwise cause the edges of the packaging material to overlap and adhere to each other without resorting to any welding and therefore expose the pack to a heat source thus causing the retraction of the film so that said film conforms to the shape of the article to be packaged or remains stretched between the edges of the support to which it has been sealed.

These films must serve the dual purpose of giving the package an attractive appearance for the consumer and at the same time ensuring, thanks to their mechanical resistance properties, that the product is protected from contamination from the external environment. Shrink films are generally produced by extrusion or coextrusion of molten polymers into thick films, rapid cooling to prevent or retard the crystallization of the thermoplastic material (s) used and orientation of the thick film by stretching, either in one direction or preferably in two. orthogonal directions, in temperature conditions in which the orientation of the polymer molecules is obtained without tearing the film. The film, of thinner thickness, thus oriented is then cooled and when it is subsequently heated to a temperature close to the temperature to which it was oriented, it will tend to shrink to recover its original dimensions.

Polymers can be extruded or co-extruded through a ring extrusion head giving a tubular often called primary tubular ("primary tape" in English), which is immediately and rapidly cooled, generally to room temperature, by means of a water bath. or a cascade of water, then heated to the orientation temperature and stretched biaxially at this temperature, for example by the technique of the air bubble (or "trapped bubble" from the English "trapped bubble"). internal pressure of a gas such as air to expand the diameter of the primary tubular to give a larger bubble and the advancement of the bubble at a speed higher than that of extrusion to obtain orientation in the transverse and longitudinal directions respectively. Generally the stretching is in a ratio of at least about 3 in each direction. The film is then cooled and cold-wound into a reel in order to maintain the shrink properties. The temperature range within which the orientation is carried out generally depends on the type of polymers used and the intended final application of the film. The orientation temperature used for the production of heat-shrink films is in any case lower than the melting temperature of at least one of the polymers present in the film itself.

Alternatively, the heat shrink films can be produced by extrusion or co-extrusion of the polymers through a flat extrusion head in the form of a web, and after a step of abrupt cooling, heating of the web to the orientation and stretching temperature. The longitudinal orientation in this case is generally obtained by sliding the film on at least two series of driving rollers where the second group rotates at a speed higher than that of the first. The transverse or perpendicular orientation is generally carried out in a "tenter frame" where the edges of the belt are blocked by clamps carried by two continuous chains that slide on two tracks that gradually spread apart as the film advances. As an alternative to the sequential stretching described above, be it longitudinal first and transversal then or vice versa transversal first and longitudinal then, the stretching can also be simultaneous in both directions. The oriented film is then cooled and re-wound according to conventional procedures.

Also in the case of orientation by means of "tenter wefts" the stretching ratio is at least 3 in each of the two orthogonal directions, but higher stretching ratios are also common.

More commonly, however, in the case of orientation in "tenter trame", the process provides for a phase in which the oriented film kept under tension is heated and thus thermofixed ("heat set" in English) so as to make it dimensionally stable to heating. For example, the orientation in "tenter frame" with heat setting is the method used industrially for the production of bi-oriented polypropylene (BOP), a film that has no heat-shrink properties.

Polypropylene-based heat-shrink films are however known in the literature.

For example, BE-A-888.914 describes a heat shrink film comprising (a) a base layer of homopolymer polypropylene and (b) a surface layer fixed to the base layer comprising a blend of 50-100% by weight of ethylene-propylene copolymer and 50-0% by weight of homopolymer polypropylene.

EP-B-126,640 claims a stretched polypropylene-based composite film comprising a base layer (a) consisting of a crystalline polypropylene and a sealing layer (b) consisting of a statistically distributed ethylene-propylene copolymer ("random" in English ) and / or an ethylene-propylene-butene-1 terpolymer with statistical distribution, characterized in that the sealing layer consists of (i) a crystalline ethylene-propylene copolymer with statistical distribution containing from 1 to 5% by weight of polymer chains having a molecular weight <6,000 and 3 to 15% by weight of ethylene or (ii) a statistically distributed crystalline ethylene-propylene-butene-1 terpolymer containing 1 to 5% by weight of polymer chains having a molecular weight < 6,000 and 1 to 10% by weight of ethylene and 4 to 30% by weight of butene-1.

These films are highly valued for their optical and mechanical properties (high stiffness) but have the disadvantage of having limited shrink properties and requiring a high shrink temperature. In fact, polypropylene, precisely because of its high modulus, is unable to impart to the film remarkable heat-shrink properties; moreover, it requires a sufficiently high orientation temperature in order to obtain orientation of the molecules without tearing the film during orientation. This is reflected in a low percentage of shrinking of the film when it is heated to the temperatures conventionally used in the packaging process, in particular in the packaging of fresh food products where the packaged product is passed through a shrink tunnel heated to temperatures generally not higher. at 140-150 ° C.

Alternatives are suggested in the literature such as for example in EP-B-282,282 where a multilayer shrink film is claimed with an inner layer of a flexible propylene resin, preferably a blend of a flexible polypropylene with a hydrocarbon resin, with an equal Vicat softening point. or below 115 ° C, and / or a VLDPE, and two outer layers comprising a flexible polypropylene with a Vicat softening point higher than that of the inner layer resin. In any case, the choice of a polypropylene resin with a softening point lower than 115 ° C for the inner layer extremely limits the range of polymers available and makes such a solution not easy.

Desirable properties in multilayer shrink films generally include not only shrink properties, but also weldability, optical and mechanical properties. Although multilayer films have been described in the state of the art that possess some of the properties indicated above to a satisfactory degree, there is always the need to search for films that have improved characteristics compared to known materials or that are particularly suitable for specific applications.

It is therefore a first object of the present invention to provide a film for packaging having optical and mechanical characteristics at least comparable with those of already known polypropylene-based films but having better heat-shrink characteristics.

It is a further object of the present invention to provide a film for packaging having optical and mechanical characteristics at least comparable with those of already known polypropylene-based films, with better heat-shrinking characteristics, and having greater planarity and a more controlled thickness, particularly suitable for use in high speed packaging machines.

It is a further specific object of the present invention to provide a film for packaging having optical and mechanical characteristics at least comparable with those of already known polypropylene-based films, with better heat-shrinking characteristics, and with improved self-adhesion properties particularly suitable for application in the packaging of paper rolls.

DEFINITIONS

For purposes of the present invention the phrase "core layer" or "inner layer" refers to any layer of the film that has both of its two main surfaces adhering to other layers of the multilayer film.

For purposes of the present invention the phrase "outer layer" refers to any layer of the film that has only one of its two main surfaces adhering to another layer of the film.

As used herein the term "sealing layer" refers to an outer layer of the film which participates in the sealing of the film to both the same layer and another outer layer of the same film, and to an outer layer of another film which ultimately ad another article that is not a thermoplastic film. The term welding also includes the adhesion between two parts of said layer due to static phenomena.

For the purposes of the present invention the phrase "free shrink" refers to the percentage change in the size of a sample of 10cm x 10cm of film when it is shrinked at a specific temperature. The quantitative determination is carried out according to the ASTM D 2732 method, as indicated in the 1990 edition of the Annual Book of ASTM Standards, Volume 08.02, pages 368-371. As used herein the term "shrink" refers to a film having a free shrink of at least 15% in at least one direction when heated to a temperature of 140 ° C for 4 seconds in accordance with the aforementioned ASTM method.

For the purposes of the present invention the term "longitudinal direction" or "machine direction" abbreviated to MD refers to the direction along the length of the film, i.e. the direction of the film as the film is formed by extrusion. The term "transverse direction" abbreviated to TD instead refers to the direction perpendicular to the machine direction.

The draw ratios are generally expressed in terms of the draw ratio in one direction multiplied by the draw ratio in the other direction, such as 3X3, 3X5, etc.

The term "corona treatment" indicates that the surface of the film is subjected to a corona discharge treatment, that is a treatment that provides that the layer of gas, such as air, in contact with the surface of the film is ionized by means of a discharge at high voltage causing oxidation phenomena and other changes on the surface of the film itself. The corona treatment of polymeric materials is described for example in US-A-4,120,716 which reports the improvement of the adhesion characteristics of the surfaces of polyethylene material caused by the corona treatment which oxidizes the polyethylene surface.

As used herein the term "polymer" refers to the product of a polymerization reaction and includes homopolymers, copolymers, terpolymers, etc.

The term "homopolymer" refers to a polymer resulting from the polymerization of a single monomer, ie a polymer consisting essentially of the same type of repeating unit.

The terms "copolymer" and "terpolymer" refer to polymers formed by the polymerization reaction of two or three different comonomers, respectively.

For the purposes of the present invention, the copolymers are identified by indicating the monomers from which they are formed.

The term "ethylene-propylene copolymer" or "EPC" therefore identifies a copolymer produced by copolymerization of propylene with ethylene, where the predominant quantity is of units derived from propylene and there is a smaller quantity of units derived from thylene.

The term "ethylene-propylene-butene-1 terpolymer" or "EPB" refers to terpolymers which incorporate these three comonomers in different percentages.

Preferably, for the purposes of the present invention, these terms mean the so-called copolymers and terpolymers with statistical distribution ("random" in English).

As used herein, the term "major amount" when referring to a polymer in a layer of the film, means that the given polymer represents at least 50% by weight of the total weight of the considered layer, while the term "minor amount" stands for indicating that the given polymer represents less than 50% by weight of the total weight of the layer considered.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a multilayer shrink film comprising an inner layer comprising a greater amount of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a blend of these and two outer layers, having a composition different from that of the inner layer, each of which comprising a greater quantity of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these, optionally mixed with a smaller quantity of a homopolymer polypropylene.

In said external layers the quantity of homopolymer polypropylene which may possibly be present is preferably comprised up to about 40% by weight. Even more preferably said amount is comprised between about 5 and about 30% by weight and even more preferably between about 10 and about 20% by weight.

The inner layer may also contain a minor amount of a homopolymer polypropylene which will range up to about 40% by weight, preferably up to about 30% by weight and even more preferably up to about 20% by weight.

According to a preferred aspect of the invention, said inner layer is substantially constituted by an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these, or by a mixture of a greater quantity of an ethylene-propylene copolymer, a ethylene-propylene-butene-1 terpolymer, or a blend thereof, with a minor amount of a polypropylene homopolymer. Other internal layers may be present in the film according to the present invention, including adhesive layers, barrier layers, added layers to increase the thickness of the film, etc.

According to a preferred aspect, however, the invention provides a three-layer film consisting of an inner layer and two outer layers as defined above. Preferably the film according to the present invention is symmetrical or substantially symmetrical. The term "substantially" herein means that the symmetrical layers may differ slightly from each other due to the presence of additives, such as antioxidant agents, sliding and anti-blocking agents, anti-condensation agents and other additives that can be added in any layer, in particular in the outer layers, in different percentages.

In another preferred aspect the invention provides a five layer film. Also in this case preferably the film will be symmetrical or substantially symmetrical with an inner layer and two outer layers as defined above where the inner layer will be comprised between two other inner layers. In a preferred aspect such inner layers will contain recycled material of the entire film. Alternatively, as in the case of a three-layer film, the recycled material, if used, can be conveniently introduced into the central layer.

The multilayer shrink film of the present invention is typically produced by coextruding a thick tape or tubular, followed by rapid cooling and orientation by the "trapped bubble" method when coextrusion is done through a ring extrusion head, or, preferably, by means of a "tenter frame" in particular when the coextrusion is carried out through a flat extrusion head.

Coextrusion through a flat extrusion head followed by orientation via tenter frame represents a preferred production method of the film of the invention since the film thus obtained will have a greater flatness and a more controlled thickness than that obtained with the bubble method. air and this will allow the use of the film on high speed packaging machines making it particularly useful for certain types of applications.

A further object of the present invention is therefore a multilayer heat-shrink film comprising an internal layer comprising a greater quantity of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these and two external layers, having a composition different from that of the inner layer, each of which comprising a greater quantity of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these, possibly in a mixture with a smaller quantity of a homopolymer polypropylene, when produced by coextrusion through a flat extrusion head and orientation via tenter frame.

The film of the present invention can be subjected to a corona treatment to improve its self-adhesion characteristics and make it particularly suitable for certain types of applications, such as the packaging of rolls of wallpaper or gift paper, where the packaging material is not heat-sealed. on itself before subjecting the package to the heat-shrinking step, but the edges of said material are simply superimposed and made to adhere to each other. This corona treatment can be conveniently carried out up to a surface energy level of about 70 dynes / cm and preferably up to about 60 dynes / cm.

A further specific object of the present invention is therefore a multilayer heat shrink film comprising an internal layer comprising a greater quantity of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these and two external layers, having a different composition from that of the inner layer, each of which comprising a greater quantity of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these, possibly mixed with a smaller quantity of a homopolymer polypropylene, corona treated .

For the types of applications where polypropylene-based heat shrinkable multilayer films are normally used, preferred thicknesses generally reach up to about 40 microns, preferably between about 10 and about 35 microns and even more preferably between about 12 and about 25 microns.

The ratio of the thickness of the different layers will depend on the total thickness of the film and the number of layers. There are no particular limitations, since the thickness of the external layers can be even about 1 micron. By way of example, however, in a symmetrical three-layer film with a thickness of up to 25 microns, an optimal ratio between the layers can range from 1/10/1 to 5/1/5.

The following examples represent preferred aspects of the shrink films according to the present invention.

The following resins were used in the production of the films of the following examples:

PP1 xHC 101 F produced by Borealis - homopolymer polypropylene with d = 0.903 g / cc and MFI = 3.35 g / 10 '(ASTM D1238 - 230 ° C, 2.160 kg)

PP2 Profax PD-064 manufactured by Montell - homopolymer polypropylene with d = 0.904 g / cc and MFI = 3.0 g / 10 '(ASTM D1238 - 230 ° C, 2.160 kg)

EPC Eltex P KS409 produced by Solvay - ethylenepropylene copolymer with statistical distribution containing 3.2% by weight of ethylene - Vicat softening point = 120 ° C (ASTM D1525) EPB1 Eltex PKS 309-X 4752 produced by Solvay - ethylene propylene-butene-1 terpolymer with statistical distribution containing about 2% by weight of ethylene and about 7% by weight of butene-1 - Vicat softening point = 113 ° C (ASTM D1525)

EPB2 Adsyl 7131 XCP produced by Montell - statistically distributed ethylenepropylene-butene-1 terpolymer containing about 3% by weight of ethylene and about 6% by weight of butene-1 - Vicat softening point = 120 ° C (ASTM D1525)

Example 1

A three-layer film A / B / A is prepared, in a 1/5/1 ratio, having a thickness of 20 microns, where A is a mixture of EPC (87.5% by weight) and PP2 (12.5% by weight) containing sliding and anti-blocking agents, and B is EPC, by coextrusion through a flat extrusion head, at a head temperature between 200 and 240 ° C, followed by sequential orientation in tenter wefts (stretch ratios: 5X8.3, MDxTD) at a temperature of about 90 ° C (MD orientation) and about 135 ° C (TD orientation).

Comparative Example 2

The film of this Comparative Example was produced following substantially the same procedure described in Example 1 but replacing the EPC in the inner layer B with PP1 and bringing the orientation temperature MD to about 120 ° C and the orientation temperature TD to about 145 ° C.

Example 3

The film of this Example was prepared following exactly the same procedure as in Example 1 but by replacing the EPC in the outer layers A with EPB1.

Comparative Example 4

The film of this Comparative Example was produced following substantially the same procedure described in Example 3 but by replacing the EPC in the inner layer B with PP1 and increasing the orientation temperature MD to about 120 ° C and the orientation temperature TD to about 145 ° C.

Example 5

A three-layer film A / B / A is prepared in a 1/5/1 ratio and a thickness of 20 microns where A is a mixture of EPC (87.5% by weight) and PP2 (12.5% by weight) containing sliding and anti-blocking agents, and B is a blend of EPC (60% by weight) and PP1 (40% by weight), by coextrusion through a flat extrusion head, at a head temperature between 200 and 240 ° C, followed by sequential orientation in tenter wefts (stretching ratios: 5X8.3, MDxTD) at a temperature of about 92 ° C (MD orientation) and about 142 ° C (TD orientation).

Example 6

The film of this Example was prepared following substantially the same procedure as in Example 5 but by increasing the percentage by weight of EPC in the blend of the inner layer B from 60 to 80 and decreasing that of PP1 from 40 to 20.

Example 7

A three-layer film A / B / A is prepared in a 1/5/1 ratio and a thickness of 20 microns where A is EPB2 containing sliding and anti-blocking agents, and B is a mixture of EPC (70% by weight) and PP1 (30 % by weight), by coextrusion through a flat extrusion head, at a head temperature between 200 and 240 ° C, followed by sequential orientation in tenter wefts (stretch ratios: 5X8.3, MDxTD) as indicated in the Example 5.

Comparative Example 8

The film of this Comparative Example was produced following substantially the same procedure described in Example 5 but using only PP1 for the inner layer B and increasing the orientation temperature MD to about 120 ° C and the orientation temperature TD to about 145 ° C.

The following assay methods were used to evaluate these films:

% of linear free retraction: the percentage of free retraction, i.e. the rapid and irreversible reduction of the original dimensions of a sample brought to a given temperature in conditions in which there is no impediment to retraction, expressed as a percentage of the original dimensions, has been measured by the ASTM D2732 method, by immersing the samples of the films to be tested (10 cm x 10 cm) for 4 seconds in an oil bath heated to 140 ° C. The percentage of free retraction was measured in both the longitudinal (LS) and transverse (TS) directions.

Opacity ("Haze" in English): opacity or haze is defined as the percentage of transmitted light that is dispersed as it passes through the sample and is measured with the ASTM D 1003 method (Procedure A).

Gloss: The gloss of the film, ie the light reflectance of a film sample, was measured using the ASTM 2457-90 method at an angle of 60 °. The percentage of free shrinkage at 140 ° C, the opacity and gloss of the films of the previous Examples and Comparative Examples are shown in Table I below

The films of Examples 1, 3, 5, 6, and 7 were also evaluated for flatness. The main flatness defects generally consist of "reliefs and hollows" ("bag and sag" in English) and "snaking" (English). For these films it was found that both the mean of the hollows and that of the deviations from the straight line, index of the "winding", were less than 25 mm. The film of Example 7 was used in a horizontal packaging machine ("Horizontal Form Fili Seal" or "HFFS" in English) MAC 200 (produced by the company Otem) with a temperature of the longitudinal sealing bar of 160-175 ° C and a temperature of the transverse sealing bar of 170 ° C, varying the operating speed of the machine from a minimum of 30 meters per minute to a maximum of 41 meters per minute. The products thus packaged were then passed through a heat-shrinking tunnel heated to 150 ° C for 4 seconds. Up to the maximum tested speed there was no problem of jamming of the packaging machine or of sealing the packages which, at the end of the test, proved to be all in specification.

Example 9

The film of Example 7 was subjected to corona treatment at a surface energy level of 42 dynes / cm. This film was then used on a MAF machine, with a linear speed of 10 meters per minute, in the packaging of wrapping paper rolls where the edges of the film around the rolls were simply overlapped and adhered to each other by virtue of the corona treatment. carried out. After passing through the shrink tunnel at about 200 ° C, the packages were evaluated both by simple visual inspection and by twisting the package, with more than satisfactory results.

Example 10

A three-layer film A / B / A is prepared in a 1/5/1 ratio and a thickness of 20 microns where A is EPB2 containing sliding and anti-blocking agents, and the internal layer B is made up of recycled material from the film of Example 7 , using the same procedure described in Example 7.

Example 11

A five-layer film A / C / B / C / A is prepared in the ratio 1/2/3/2/1 and thickness 20 microns where A and B are as defined in Example 1 and C is made up of recycled material of the film of Example 1, using the same procedure described in Example 1.

Example 12

A five-layer film A / C / B / C / A is prepared in the ratio 1/2/3/2/1 and thickness 20 microns where A and B are as defined in Example 1 and C is made up of recycled material of the film of Example 11, using the same procedure described in Example 1.

Example 13

A three-layer film A / B / A is prepared in 1/3/1 ratio and 15 micron thickness where A is EPB2 containing sliding and anti-blocking agents, and B is a mixture of EPC (70% by weight) and PP1 (30 % by weight), by coextrusion through a flat extrusion head, at a head temperature between 200 and 240 ° C, followed by sequential orientation in tenter wefts (stretch ratios: 5X8.3, MDxTD) as indicated in the Example 5.

Example 14

A three-layer film A / B / A is prepared in a 1/3/1 ratio and 12 micron thickness where A is EPB2 containing sliding and anti-blocking agents, and B is a mixture of EPC (70% by weight) and PP1 (30 % by weight), by coextrusion through a flat extrusion head at a head temperature between 200 and 240 ° C, followed by sequential orientation in tenter wefts (stretch ratios: 5X8.3, MDxTD) as indicated in Example 5 and it is subjected to corona treatment at a surface energy level of 45 dynes / cm.

Claims (9)

CLAIMS 1. Multilayer shrink film comprising an inner layer comprising a greater amount of an ethylene propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these and two outer layers, having a composition different from that of said inner layer , each of which comprising a greater amount of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a blend of these, optionally in a blend with a smaller amount of homopolymer polypropylene. 2. The film of claim 1 wherein at least one of the outer layers contains up to about 40% by weight of polypropylene homopolymer. 3. The film of claim 2 wherein at least one of the outer layers contains from about 5 to about 30% by weight of polypropylene homopolymer. 4: The film of claim 3 wherein at least one of the outer layers contains from about 10 to about 20% by weight of polypropylene homopolymer. 5. The film of any one of the preceding claims characterized in that said inner layer is substantially constituted by a greater quantity of an ethylene-propylene copolymer, an ethylene-propylene-butene-1 terpolymer, or a mixture of these, possibly in a mixture with a minor amount of homopolymer polypropylene. The film of any one of the preceding claims characterized by being symmetrical or substantially symmetrical. 7. The film of claim 6 characterized by having 3 layers. The film of any one of the preceding claims when obtained by extrusion through a flat extrusion head and orientation by "tenter frame". The film of any one of the preceding claims characterized in that it has been subjected to a corona treatment.